The present invention relates to a dopant source for diffusion doping of a semiconductor substrate or, more particularly, to a source of a p-type dopant for doping of a semiconductor substrate by the method of diffusion.
Several methods are known and practiced in the prior art for doping a semiconductor substrate with a dopant by the method of diffusion including: (a) the gas-diffusion method using a gaseous dopant as the diffusion source; (b) the ion plating method by bombarding the semiconductor substrate with ions of a dopant element under high vacuum; and (c) the solid-diffusion method using a sintered body of a metal oxide or metal nitride. The last mentioned solid-diffusion method is usually preferred to the first and second methods in respect of the advantages that high uniformity is obtained in the concentration of the dopant over the substrate surface, the apparatus used for the method is relatively simple and inexpensive, the method is suitable for mass production of semiconductor devices and the method can be performed without using any toxic or dangerous gas such as diborane.
The dopant source for the solid-diffusion method is usually in the form of a thin disc similar to semiconductor silicon wafers prepared by sintering a fine powder of the metal oxide or metal nitride. A problem in such a solid dopant source in the form of a sintered disc is that the sintered body is more or less unavoidably contaminated with various impurities or, in particular, metallic impurities originally contained in the powder of the metal oxide or nitride subjected to sintering and originating in the binder used to facilitate sintering when it is a metallic compound. When a semiconductor substrate of silicon or germanium is desired to be doped with a p-type dopant, for example, the most widely used solid dopant source is a sintered body of boron nitride while boron nitride powders used for sintering usually contain several kinds of impurities such as sodium phosphate, sodium oxide, iron oxide, calcium oxide and carbon as an unavoidable consequence of the method for the preparation of the boron nitride powder. Further, boron nitride powders are sintered usually with admixture of boric anhydride, calcium oxide, aluminum oxide, sodium oxide, aluminum phosphate or silicon dioxide as a binder to ficilitate sintering so that the sintered body of boron nitride always contains these binder materials remaining therein. The above mentioned impurities in the sintered boron nitride body as the solid dopant source originating in the boron nitride powder per se and the binder might cause no serious problems at least some years ago when the semiconductor substrates had small and thin dimensions and no extremely high performance was required for the doped semiconductor devices.
Along with the trend of requirements in recent years toward larger and larger diameters of the semiconductor substrates and higher and higher performace of semiconductor devices, however, the influences caused by the impurities in the solid dopant source are no longer negligible and, according to the inventors' knowledge obtained by the investigations, the above mentioned impurities in the solid dopant source of boron nitride have profound influences on the crystalline dislocations and lattice defects in the semiconductor substrates doped therewith.